Abstract
This study shows the neotectonic deformation occurred in the southern piedmont of the Cumbres Calchaquíes, in the Amaicha and Tafí valleys. Neotectonic deformation manifests itself through faults, folds and diversions of drainage channels. The Amaicha valley is bounded to the north by the Tafí del Valle fault and to the south by the Los Cardones fault. The Cumbres Calchaquíes ride over the Sierra de Aconquija through the Los Cardones and Carapunco faults. The Carapunco fault also has a synestral component, responsible for generating an imbricated system of contractional fractures. In the study region many earthquakes of ≥ 3 and ≥ 4 magnitude coincide with regional faults evidencing its neotectonic activity. The seismic energy dissipated through materials with less cohesion that form the fill of the valleys, generating discrete fault scarps and strongly folded conglomerate strata. The foothills deposits of the Cumbres Calchaquíes absorbed most of the seismic energy released during the reactivation of the faults. Tectonic activity is deforming 630 a BP deposits in the Cumbres Calchaquíes piedmont.
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1 Introduction
The deformation of the Andean foreland by neotectonic processes has been treated extensively in the national and international literature. This evidences the interest of the scientific community in understanding tectonic rotations, direction of tectonic shortening, depths of detachment of the main structures, position of the front deformation, vergence of the structures, seismic activity associated with the movements of the faults and deformations of the mountain fronts.
The Tafí and Amaicha valleys are intramontane tectonic basins located in the province of Tucumán, Argentina, in the northern end of the Sierras Pampeanas (Fig. 1). The Tafí valley is located in the northeast end of the Sierra de Aconquija, bordered to the north by the Cumbres Calchaquíes. In the center of the valley, the independent relief of Loma Pelada stands out, dividing the valley into two related areas, Las Carreras valley to the west and Tafí valley itself to the east (Fig. 2). The bottom of the Tafí Valley is between 1800 and 2300 ma.s.l., crossed from north to south by the Tafí del Valle river. The Amaicha valley, with NW strike, extends to the northwest of the Tafí valley, from the Infiernillo pass to the Santa María valley, limited to the north by the Cumbres Calchaquíes and to the south by the Sierra de Aconquija (Figs. 1, 2).
Regional location of the study area. Morphotectonic map to show the major orographic units and the regional structures that limit them. Q: Fluvio-eolian deposits, Quaternary. VN: Volcanic rocks, Neogene. SN: Neogenic sediments of the Tucumán basin and the Choromoro valley. SMN: Santa María Group, Neogene. SBP: Santa Bárbara Subgroup, Salta Group, Paleogene. PK: Pirgua Subgroup, Salta Group, Cretaceous. BMP: Proterozoic early – Paleozoic Basement. 1: Lineaments. 2: Normal Fault. 3: Reverse Fault. 4: Transcurrent Fault. 5: Study area. 6: Fig. 4 location. 7: Seism M ≥ 3. 8: Seism M ≥ 4. 9: Seism M ≥ 5. SPdC: San Pedro de Colalao. AR: Amaicha River. RF: Rearte Fault. TVF: Tafí del Valle Fault. TV: Tafi Valley. SJR: San Javier Range. LCF: Las Cañas Fault. SR: Salí River. NF: Narváez Fault. F3: Fig. 3 location
Morphotectonic map of the Tafí and Amaicha valleys. It shows the depressions of Tafí and Amaicha and the regional structures that gave rise to them. In addition, they do indicate the larger geological units that condition the fracturing that gave rise to the valleys. On this scheme, the recently generated structures that are deforming the foothills were drawn with red lines. Q: Fluvio-eolian deposits, Quaternary. YN: Yasyamayo Formation, Neogene. VN: Volcanic rocks, Neogene. LCN: Los Corrales Formation, Neogene. AN: Andalhuala Formation, Neogene. SP: Saladillo Formation, Paleogene. GrPz: Paleozoic granites. BMP: Proterozoic early – Paleozoic Basement. 1: Lineaments. 2: Normal Fault. 3: Reverse Fault. 4: Transcurrent Fault. 5: Recent structures. 6: Bedding with dip indication. AF: Aconquija Fault. LCF: Los Corpitos Fault. LAD: La Angostura Dam. OF: Ovejería Fault. PR: Pelado River. AA’: Profile Fig. 5. BB’: Profile Fig. 8. CC’ Profile Fig. 9. F7: Fig. 7 location. F3b: Fig. 3b location
Studies carried out in the Tafí and Amaicha valleys have not addressed the valley's tectonics. Until now, no faults or folds had been recorded in these valleys. Even the geomorphological studies did not deal in detail with the landscape deformation, deviation of the drainage channels, backward erosion, seismic activity, topographic unevenness of the foothills, etc.
So far there is no agreement on regional faults in the Amaicha valley. Ruiz Huidobro [1], Ferreiro and Mon [2], González [3] and González and Barreñada [4] considered that the Amaicha depression was delimited by fault on both edges. With another interpretation, Gutiérrez et al. [5], Gutiérrez and Mon [6] and Mon et al. [7] considered that only the Tafí del Valle fault separated the Cumbres Calchaquíes from the Sierra de Aconquija, with a normal component sloping to the SW and another dextral horizontal slip. The Tafí del Valle fault [8] would have disposed the Cumbres Calchaquíes on the Sierra de Aconquija [4]. The classic scheme proposed by González Bonorino [9] considers that the basement blocks were raised by large lystric reverse faults.
Our work shows the differential deformation in a mountain front as a result of the propagation of seismic energy through foothills deposits. In this analysis, the cohesion of the geological material is considered relevant for the propagation of the seismic energy produced by the activity of the faults. We take as a study case the neotectonic activity in the Tafí and Amaicha valleys and the structural relationship between the Cumbres Calchaquíes and Sierra de Aconquija. We show the structures and morphologies generated in the southern foothills of the Cumbres Calchaquíes. These structures with vertical and horizontal components occur in foothills, close to regional structures on the mountain front. Previous theories related to the existence or not of two faults in the Amaicha valley are also discussed and field data are presented.
2 Methodology
The research was carried out based on the collection of bibliographic antecedents, interpretation of satellite images and field data surveys. To develop thematic cartography, the visual and digital interpretation of satellite images LANDSAT 8, SENTINEL 1, RADARSAT were carried out. The digital treatment for the visual interpretation of the LANDSAT and SENTINEL images consisted of the combination of different bands. All satellite and radar images were interpreted using the QGis software and then thematic maps were drawn. To prepare the topographic profiles we used high resolution digital elevation models (12.5 m) provided by the ALOS PALSAR mission. During field work, the surfaces deformed by neotectonic processes were identified (fault steps, landslide zones, deformed foothills, whale-back morphologies in alluvial fans, erosion of conglomerate terraces and transport of rolling by surface drainage, development of drainage networks as indicators of deformation) and data on the structures that generated these morphologies were collected.
3 Regional tectonic morphology
In the plate tectonics context, subjected to compressive stress with NE strike during the Cenozoic, the study area coincides with the latitude where the Nazca plates change the angle dip, high angle north of 27° S and low angle at south [10,11,12].
3.1 Morphology
The Sierra de Aconquija and Cumbres Calchaquíes seem to have formed a single mountain block with a NNE strike and slightly curved, concave towards the west. It have been raised by high-angle, double vergent, reverse faults on both edges [13, 14], Cumbres Calchaquíes and Aconquija faults on the western edge and Periquillo fault on the eastern edge (Fig. 1).
Probably in the Pleistocene the drainage system developed on the eastern edge of the Cumbres Calchaquíes, Sierra de Aconquija and Ambato block had an approximate general direction towards the east and ended in the Chaco plain, covering a large area of surface runoff, maintaining the contribution of coarse sediments by the continuous uplift of the mountain system and appropriate climatic conditions [5]. The records of these hydrogeological processes are the thick conglomerates deposits arranged in the upper parts of the terraces of the Los Sosa River, which were eroded by the current surface runoff. Later, with the progress of tectonic activity, this drainage system completely eroded, configuring the current network [5]. The Tafí valley is independent from the Amaicha valley, the water sub-basins of both valleys have their springs in the Infiernillo pass (Figs. 1, 2). The waters of the Amaicha valley are part of the Plata basin through the Santa María, Las Conchas and Salado rivers, and the waters of the Tafí valley end in the Salí - Dulce river, forming part of the endorheic basin of the Mar Chiquita lagoon [15,16,17].
3.2 Regional geology
These ranges are made up of an igneous—metamorphic basement (Upper Proterozoic—Lower Paleozoic) [18, 19] (Fig. 1). Sediments from the Cretaceous Rift could not have covered these mountain ranges because they were elevated [7], the outcrops of the Pirgua Subgroup of the Salta Group reached the northern end of the Cumbres Calchaquíes and the western edge of the Sierra de San Javier (Fig. 1).
Neogene sediments occupied the Choromoro and Las Estancias basins in Tucumán and the Santa María basin in Catamarca. In the Santa María valley the sedimentary sequence overlaps the metamorphic basement towards the Amaicha valley, reaching the Infiernillo pass [7, 20, 21], while in the northern end from the Sierra de Aconquija the volcanism that gave rise to the extrusive rocks of Cerro Las Ánimas occurred ca. 13 Ma [22, 23]. The sedimentary column of the Santa María valley was ordered in Sequences by Bossi et al. [15] (Table 1).
In the Tafí valley, on the igneous-metamorphic basement [32] rest thick grayish-brown conglomerates, called Lomitas Pegadas Formation [33], covered by loess from Tafí del Valle Formation [34], dated at 1.15 Ma (Middle Pleistocene) by Schellenberger et al. [35] (Table 1).
Organic matter contained in lake sediments in the area of El Rincón, Tafí del Valle, were dated with 14C, registering ages between 10,350 ± 80 and 4120 ± 60 a BP [36]. Ortíz and Jayat [37] carried out radiocarbon dating on bone remains found in the Tafí del Valle Formation and located it at the Pleistocene-Holocene limit (10.25–9.65 ka cal. BP). Recent studies carried out an evolutionary geomorphological model of the Lateglacial and Holocene accumulations in the valleys determining four degradation stages dated between 13,000 and 630 a BP with 14C, thermo Luminiscence and tephra layers underlying charcoal samples [30, 31, 38] (Table 1).
3.3 Regional structures
Marrett et al. [39] document for northwestern Argentina two kinematic regimes that characterize the late Cenozoic deformation, a WNW shortening and a strike-slip phase with ENE shortening. Riller and Oncken [40] consider that the segmentation of the upper crust into rhomb-shaped domains confined by transpressive ranges resulted from the decrease in crustal shortening with distance to the central bend of the Andes and Strecker et al. [41] and Mon [42] agree that the faults that raise the edges of the mountain ranges could be reactivated Paleozoic structures. Currently, the Sierra de Aconquija—Cumbres Calchaquíes mountain range is separated at the western edge where it opens towards the Santa María valley through the Amaicha valley (Fig. 1). On the eastern edge it remains together, but both mountains are unlinked by the Tafí del Valle fault (Fig. 1).
The Tafí del Valle fault zone has evidence of variations along strike proved by the interpretation of satellite images, surface geometry and the rock outcrops in the Tafí and Amaicha valleys. In the southeastern end it is a single plane that separates the Sierra de Aconquija from the Cumbres Calchaquíes. It was possible to measure the fault plane in a stream, at the entrance to the Tafí valley, registering the following values (225°/88°, 228°/85°) (Figs. 2, 3a). In the north end of the Tafí valley, at the foot of the fault, a wide deposit of piedmont develops that reaches 23 km in length and extends between 2 and 4 km in width. It is assumed that these deposits are arranged on a metamorphic basement hiding the fault plane (Fig. 2). In the Amaicha valley, the fault zone is about 6 km wide, hosts the upper sequence of the Santa María Group and opens towards the Santa María valley, where the dextral transcurrent geometry of the fault was verified [7] (Fig. 2).
a View of the Tafí del Valle fault with normal and dextral transcurrent displacement components. The hammer indicates the fault plane, high angle. It marks the limit between the Sierra de Aconquija and the Cumbres Calchaquíes. b Reduced outcrop of sandstones and conglomerate siltstones of the Santa Bárbara Subgroup, Salta Group, on both banks of the Los Sosa River, southern end of the Chasquivil fault. The sedimentary strata dips to the southeast (160°/40°) tilted by the Chasquivil fault. The location of the photo can be seen in Fig. 1
Internally, the mountainous ensemble is also cut by regional NNE structures (Infiernillo, Portugués, Chasquivil faults and Los Pinos lineament), which do not manage to completely separate them, but give the ensemble a morphology that allows identifying minor mountain ranges such as Las Lajas, Las Pavas and Altos de la Totora, which, had the initial deformation continued, would have formed independent units, as is the case with the San Javier range (Fig. 1). In some cases, these structures gave rise to important rivers such as the Amaicha, Tafí del Valle—Los Sosa and Las Cañas rivers and, other times, they separate mountain blocks, such as the Los Pinos lineament that separates the Las Lajas and Santa Ana mountains from the Sierra de Aconquija (Fig. 1).
The central zone of the Cumbres Calchaquíes and northern end of the Sierra de Aconquija, crossed by the NNE structures, the Portuguese fault (to the west) and the Chasquivil fault (to the east), is intruded by granite bodies that constitute a strip with the same orientation as structures that limit them, where the Alto de los Cóndores, Negro, Pabellón, Loma Pelada, Ñuñorco Grande and Ñuñorco Chico hills stand out (Fig. 2).
A cortical extension was postulated in the early Miocene to explain the origin of Cenozoic basins in the northwestern Sierras Pampeanas that were inverted at approximately 5 Ma [1, 6, 15]. However, it is likely that these regional structures were partially active during the Cretaceous Rift, but did not evolve sufficiently to form depocenters. For example, the Sierra de Aconquija and Cumbres Calchaquíes remained elevated during the Cretaceous because no outcrops of this age were found in the inner valleys [7]. But, in all these regional NNE structures spaces were generated during the Cretaceous Rift where small outcrops of red sandstones and conglomerate siltstones are preserved, attributed to the Santa Bárbara Subgroup of the Salta Group (Paleogene) (Fig. 1). We can see them along the Aconquija and Cumbres Calchaquíes faults, in the Los Corpitos fault, on the eastern edge of the Los Pinos lineament and on the eastern slope of the Santa Ana mountain range, on the northeastern edge and southern end of the Chasquivil fault, in the Los Sosa riverbed and on the eastern edge of the Periquillo fault (Figs. 1, 2, 3b). These reduced outcrops of red strata attributed to the Santa Bárbara Subgroup and arranged on a metamorphic basement document that the other lower sequences of the Salta Group are not found in these regional structures. Possibly, in the Cretaceous Rift stage, the Aconquija, Cumbres Calchaquíes, Los Corpitos, Infiernillo and Portugués faults constituted incipient steps that dipped towards the west, just as the Chasquivil fault represented a step dipping to the east, and the mountainous group remained elevated (Figs. 1, 2). The granite bodies represented by the Alto de los Cardones, Negrito, Pabellón, Loma Pelada, Ñuñorco Grande and Ñuñorco Chico hills constituted a longitudinal block towards which the steps of the normal faults converged, those that inclined to the east and those that inclined to the west (Fig. 2). In the lower Tertiary these small steps began to fill up with the sediments of the Santa Bárbara Subgroup and were protected in the elevated parts of the slopes of the mountains, such as on the western edge of the Sierra de Aconquija and Cumbres Calchaquíes, in the Los Corpitos fault. [7, 22], in the Chasquivil fault and many other places (Figs. 1, 2, 3b). Against the Infiernillo fault, the upper sequence of the Santa María Group ends and filling the Amaicha valley, without reaching the Tafí valley [7].
These areas of regional weakness, Cretaceous or Paleozoic, were reactivated with the Andean tectonics. Sometimes the normal faults of the Cretaceous Rift were reactivated in the same direction as the initial fault plane, but in the opposite sense [43,44,45,46] and later on, these faults were cut by others with opposite sense and direction such as the Aconquija, Cumbres Calchaquíes, Infiernillo faults [6, 7, 22, 47]. However, the Chasquivil fault would have had a reverse reactivation because it arranged the preserved red strata in the Los Sosa River dipping 40° to the SE (Fig. 3b). Pilger [48] interpreted that the uplift of the marginal mountain ranges to the Calchaquí valley would have started at 13 Ma. Tectonic activity would have accelerated around 4 Ma, affecting the Yasyamayo Formation (ca. 1.5 Ma), therefore would have occurred at approximately 1.2 Ma, extending to at least 0.6 Ma [49] and the eastern edge of the Sierra de Aconquija began to rise at 9 Ma [50] (Fig. 2).
4 Seismic record
According to the seismic classification for Argentina, based on studies carried out until 1983, with the exception of the areas surrounding the cities of Salta and San Salvador de Jujuy, the north and a large part of the Sierras Pampeanas are located in Zone 2, of moderate danger [51]. We collect information on the seismicity around the study area, recording only earthquakes with magnitudes between 3 and 6 [52]. From 1973 to date there have been 31 earthquakes of magnitude ≥ 3 registered between 7 and 204 km depth, 9 earthquakes of magnitude ≥ 4 generated between 8 and 60 km depth and 5 earthquakes of magnitude ≥ 5 between 27 and 37 km deep (Fig. 1). We observe that earthquakes of magnitude ≥ 3 are aligned with regional structures such as the Tafí del Valle, Aconquija, Chasquivil, Periquillo, Las Cañas faults and the Los Pinos and Tucumán lineaments (Fig. 1). The earthquakes of magnitude ≥ 4 are aligned with the Tucumán and Los Pinos lineaments and the Narváez and Río Muerto faults and two of the earthquakes of magnitude ≥ 5 with the Río Muerto and Carapunco faults (Figs. 1, 2). Other recent and recurrent evidence of tectonic activity in the Tafí and Amaicha valleys and in the surrounding region are river captures [5], landslides and rock falls like on the left slope of the Los Sosa River (Fig. 4) on Provincial Route 307 (Fig. 2) and retrograde erosion.
a Azucena landslide on the left bank of the Los Sosa river (Chasquivil fault). b Fin del Mundo landslide on the left bank of the Los Sosa river (Chasquivil fault). Landslide scars are shown with red lines
5 Earthquakes effects
In the seismogenic region of the continental crust (15–18 km), earthquakes produce an episodic rigid frictional deformation [53,54,55,56] whose rupture by propagation during deformation is controlled by the anisotropic fabric at the cortical scale [57]. When an earthquake is generated, oscillations occur in existing faults or, less frequently, in new faults [56, 58], the elastic stress accumulating in the rocks surrounding the fault zone, until the resulting stress in the plane it exceeds its frictional resistance and failure occurs [59].
The conditions of the shear stress (\({\tau }_{f}\)) at which the failure is going to slip are indicated by the equation
where \({C}_{^\circ }\) is the cohesion force, \({\mu }_{s}\) is the coefficient of static friction, \({\sigma }_{n}\) is normal effort and \({P}_{f}\) is the fluid pressure [59] and high fluid pressures would favor the reactivation of faults, reducing the cohesion force [60]. However, for new faults to form in a region where faults already exist it is required that those old faults develop sufficient cohesion and/or have insufficient fluid pressure in the pores, in such a way as to prevent them from reactivating, even with favorable orientations [60]. It was also suggested that the migration of the fault activity towards the foothills is due to the cessation of activity of the faults of the mountain fronts, these new structures inheriting the style of the main faults [61]. Most earthquakes result from increased shear displacement in the fault [62] generating rupture and slip that are roughly related to the magnitude of the earthquake [63,64,65] (Table 2).
Different effects of seismic wave propagation are observed in the study area. Figure 4 shows the breakdown of granitic and metamorphic rocks in the plane of the Chasquivil fault due to the propagation of seismic waves during its reactivation. These crushed rocks, suspended on steep slopes, weather, form soils, rainwater saturates their pores and trigger landslides. New fractures were generated in the Amaicha and Tafí valleys and in the Tafí valley folds of the conglomerate strata were produced in the pedemontane deposits (Fig. 2).
6 Quaternary deformations
Numerous works document the recent tectonic activity in Geological Provinces of Argentina that occur in the foothills, in the intra- and intermontane basins or also refer to the geological processes triggered on the mountain fronts by active seismic activity, such as landslides in the slopes of the mountains, formation of dikes by obstruction of rivers, etc. The earthquake rupture zone is not located in the main fault zone of the mountain front but on the flank of the mountain range [66], in a new fault zone called FAF (frontal active fault/flexture) that it is generated due to the marked difference in stiffness between the piedmont and basement deposits [61, 67].
On the southeastern edge of the Precordillera, the neogene strata ride on quaternary deposits of the foothills of the mountain front and, in the Sierra Chica de Córdoba, Sierras Pampeanas, the most recent deformations linked to the Sierra Chica fault system are not located in the topographic break of the mountain slope, but usually appear associated with a secondary structure [61, 68,69,70]. The topographic growth in the sedimentary basins, on the eastern edge of the Sierras Pampeanas, shows the progressive deformation towards the east, obstructing and diverting surface drainage. The Alto de Mansilla is a topographic elevation just 130 m high, generated by convergent heading faults and overlapped with step-over restraining bend geometry, which divided a large lake and turned it into the Salinas Grandes and Salinas de Ambargasta [17].
In the alluvial fans of the foothills of the La Candelaria range (Santa Bárbara System) topographic highlights attributed to neotectonic activity and related to seismic events were observed [71]. On both flanks of this mountain range the geometry of the faults was determined, some blind, riding quaternary alluvial fans, making profiles with 2D electrical resistivity tomography, with seismic methods and with digital terrain models prepared with images obtained with drones. Repeated offsets during moderate-to-large earthquakes in the Quaternary are the most likely origin for these scarps, which suggests recurrent, spatially disparate seismogenic quaternary deformation processes in the broken foreland [72,73,74,75].
The sedimentary cover in the Cretaceous Rift basins is very thick (3554 m in the Metán basin, 3000 m in the Choromoro basin and 7000 m in the Tucumán basin) [45, 46, 76], so the Neogene and Quaternary deformation folded these sedimentary piles, laying them on the Proterozoic—Paleozoic basement that is exposed in small outcrops, in the upper parts of the mountains. This tectonic morphology is mainly due to the inverse reactivation of normal faults inherited from the Cretaceous Rift, sometimes blind [43,44,45,46]. In the Metán basin, recent deformation was documented in folded surfaces, associated with blind and inherited structures. A case study with deep seismic and shallow geophysical prospecting methods evidenced the deformation produced by the El Galpón fault, folding neogenic and quaternary units, which would have experienced a seismic movement of M 5.7 in 2015 (Aranda-Viana Pers. Com.).
In the Cafayate valley, the Quaternary deformation produced folds in the lake deposits from the Upper Pleistocene to the Middle Holocene, evidencing the reactivation of the main high-angle faults, verging to the west, of the eastern edge of the valley, generating secondary faults that affect the quaternary coverage [77].
7 Results
In the Tafí and Amaicha valleys (Fig. 2), in the foothills of the south end of the Cumbres Calchaquíes, the surface relief is disturbed by structures wich was generated during neotectonic activity that give the landscape a morphology that breaks the monotony of the forms generated by alluvial fans and the fluvio-aeolian erosion. These structures sometimes occur, in the piedmont, far from the pre-existing larger structures, such as the examples cited in the literature [61, 67, 78] and, other times, directly on the mountain front. The Sierra de Aconquija and Cumbres Calchaquíes mountains are limited by two faults; the Tafí del Valle fault marks the southern edge of the Cumbres Calchaquíes and the Los Cardones (Amaicha valley) and Carapunco (Tafí valley) faults the north edge of the Sierra de Aconquija (Fig. 2).
7.1 Amaicha valley
The Amaicha valley is narrow, with an average width of about 4 km (Fig. 1). The developed piedmonts belong to the southern foothills of the Cumbres Calchaquíes, it is covered by Sequences III and IV of the Santa María Group and the Quaternary alluvial fan deposits (Fig. 2; Table 1).
We consider that the Tafí del Valle fault had a normal component that separated the Sierra de Aconquija from the Cumbres Calchaquíes and formed the valleys of Tafí and Amaicha. This fault allowed the deposition of the Tertiary sequence in the Amaicha valley, therefore which results from an underdeveloped heritage from the Cretaceous Rift [7] (Figs. 1, 2, 5). Probably, the Tafí del Valle fault could have had an incipient development during the Cretaceous Rift stage, which did not prosper. Sediments of Cretaceous or Tertiary age were not yet found in the Tafí valley, possibly because the area remained elevated, until in the lower Tertiary it began to partially fill up on the Amaicha valley side, reaching the Neogene deposits at the foot of the Infiernillo fault. As an example, remnants of the lower Tertiary red sediments (Santa Bárbara Subgroupe) occur at the foot of the Los Corpitos and Chasquivil faults (Figs. 1, 2, 3b).
Geological profile AA′, transverse to the Amaicha valley. 1: Los Corrales Formation, Santa María Group, Neogene. 2: Paleozoic granite. 3: Proterozoic early – Paleozoic Basement. 4: Reverse fault. 5: Normal fault. The Los Cardones fault zone has a general dip direction to the NE, dipping 40° (Fig. 2)
Neogene sedimentary sequences of the Santa Maria Group spread eastward into the Sierra de Aconquija, against the dipping ramp to the west, which was later cut by the reverse Infiernillo fault, vergent to the NW [7]. The Infiernillo fault separeted the Amaicha Valley from Tafí Valley and would have also facilitated the magmatic ascent that originated the vulcanites in the northern zone of the Sierra de Aconquija (Figs. 1, 2). The horizontal displacement of the Tafí del Valle fault is affecting the sedimentary sequence of the Santa María Group, in the Santa María valley [7].
In the Amaicha river we measured the plane of the Los Cardones fault. It is a reverse fault that dips to the NE. Due to the fact that the fault plane in the granite outcrops is broken, it was possible to measure a plane dipping 40° to the NE (Figs. 5, 6). The Los Cardones fault cuts the Tafí del Valle fault and places the layers of the Los Corrales Formation with a 40° dip to the east (Table 1; Figs. 2, 5, 6). Southwest of the Los Cardones fault, the layers of the Los Corrales Formation slope 20° to the SE (Fig. 2).
a Photo illustrating the Los Cardones fault at the Amaicha Valley, documenting the profile in Fig. 5. The Los Corrales Formation, Santa María Group, Neogene rest on Paleozoic granites is observed. The complex is affected by the Los Cardones reverse fault that arrange the sedimentary layers of Los Corrales Formation dipping to the east. Stratification and fault plane measurements indicate DD/D. The white line is the contact between the granite and the Los Corrales Formation. Bedding in the Los Corrales Formation is marked with yellow lines. b Diagram of resulting fractures represented in Schmidt's equiareal network, lower hemisphere. Los Cardones fault in the red line. In blue lines the faults in Fig. 7
In the Amaicha valley, tectonic activity is affecting the Quaternary conglomerate sequence. It is possible to see how the layers of the Andalhuala Formation are riding on the Yasyamayo Formation and the faults also affect the quaternary gravels (Table 1; Figs. 2, 7). These faults occur in the central zone of the valley, far from the main faults that limit the mountains.
a Photographic illustration in the Amaicha Valley. A reverse fault places the Andalhuala Formation on the Yasyamayo Formation. Other faults also affect quaternary gravels. The white lines mark the contacts of the sedimentary units. Yellow lines indicate bedding. Red lines are identifying structures. On the lower right edge, in white, a person's half-length silhouette indicates scale. b Diagram of resulting fractures represented in Schmidt's equiareal network, lower hemisphere. Los Cardones fault in blue line. In red lines faults in this figure
7.2 Tafí valley
In the Tafí valley, alluvial fans are fed from both Cumbres Calchaquíes and Sierra de Aconquija. The deposits corresponding to the Cumbres Calchaquíes are better developed, cover the entire mountain front of the southern edge and reach up to 4 km wide. On the other hand, the deposits in the foothills of the Sierra de Aconquija occupy only half of the northern front of the mountain range and are 2 or 3 km wide (Fig. 2). The thickness of the Quaternary deposits at the Mesada Lamedero is estimated to be about 200 m.
Peña-Monne and Sampietro-Vatuone [30] defined in Tafí valley morphosedimentary units constituting aggradation phases. Unit H1 represents the oldest hillside accumulations, river terraces and alluvial cones, consisting of well-stratified coarse sands and gravels. Unit H2 occupies a large area constituting alluvial fan deposits made up of sediments similar to H1 (Table 1).
The differential tectonic behavior of both mountain blocks (Sierra de Aconquija and Cumbres Calchaquíes) generated the segmentation of the NW structures that limit both mountain ranges, the Tafí del Valle fault on the southern edge of the Cumbres Calchaquíes and Los Cardones—Carapunco faults on the edge north of the Sierra de Aconquija (Figs. 1, 2). This differential behavior can be observed in the Los Cardones and Carapunco faults (Fig. 2). Apparently the Los cardones fault only has dip–slip. On the other hand, the Carapunco fault has vertical and horizontal displacement. The Tafí valley reflects a differential tectonic behavior in neotectonic processes; several zones are distinguished in the foothills of the Cumbres Calchaquíes (Las Tacanas, La Costa II, Los Cuartos, Las Siringuillas and La Angostura), limited to the west by the Portuguese fault and to the east by the Chasquivil fault (Fig. 2).
7.2.1 Las tacanas
The Carapunco fault marks the northern limit of the Sierra de Aconquija, through it the river Tafí del Valle runs. The fault begins in the El Infiernillo pass and ends in the La Angostura area (Fig. 2). Towards the Tafí del Valle river, the alluvial fans of the Cumbres Calchaquíes and the Sierra de Aconquija converge.
In the Las Tacanas area, it is observed in the satellite images and in the geomorphology of the landscape, that the alluvial fans of the Cumbres Calchaquíes were reworked and modeled by other processes, different from the accumulation and erosion that characterize these geoforms. These surfaces are furrowed by deep, parallel and curved ravines that descend from the Cumbres Calchaquíes and end in the Tafí del Valle river, forming positive reliefs and hills between them (Figs. 2, 8). This drainage network with a curved and parallel pattern is represented by deep ravines generated by reverse faults that formed a series of ascending steps to the NW in the foothills (Fig. 8a). In this área, retrograde erosion and landslides on the southern slopes of the streams due reverse faults that changes the local base level, allow evidence of recent tectonic activity. These neotectonic indicators are consistent with the deformation associated with the recent seismic activity recorded in the área (Table 2).
a Topographic profile BB’ showing the positive relief and the hills formed in the foothills of the Cumbres Calchaquíes, generated by reverse faults, configuring a rough surface limited by deep ravines. b The plant diagram shows an interpretation of the geometry of interposed contractional fans drawn based on the distribution of the drainage network in Fig. 2 and the attitudes of the fault planes
In the upper parts of the foothills, the strata of a sequence of sandy silts, of a light brown color, with intercalations of conglomerate lenses, with large clasts, maintain an inclination of 18° to the south (Fig. 9a). Together, these streams and the Tafí del Valle River form an interlaced contractional fan geometry (Fig. 8b), like the examples from the literature [79, 80]. This geometry allows us to determine that the Carapunco fault has a synestral transcurrent component (Figs. 2, 8b).
a Stratigraphic sequence of the foothills on the right bank of the La Bolsa ravine. The conglomerate strata slope south (185°/18°). b Quaternary terrace on the right bank of the Blanquito Viejo river. The sequence was affected by the Blanquito Viejo fault, disposing the conglomerate strata with a west dip (265°/30°)
The Cumbres Calchaquíes are exerting pressure on the Sierra de Aconquija, generating the deformation of the foothills of the valley, in such a way that the foothills of the Cumbres Calchaquíes are higher than the foothills of the Sierra de Aconquija (Fig. 10a). This tectonic process that generates a characteristic morphology and a typical structural geometry is evidenced in the Mesada Lamedero (Fig. 10). There, a positive relief structure is formed by the Carapunco and Blanquito Viejo reverse faults, arranging the conglomerate layers of the alluvial fans of the Cumbres Calchaquíes with a strong east and west dips (Figs. 2, 9b, 10).
a Panoramic view of the Tafí Valley tectonics morphology displayed in figure b. We can see the topographic relief of the alluvial fans disturbed by the reverse faults generating the counter slope. It is also observed how the piedmonts of the Cumbres Calchaquíes rise above the Sierra de Aconquija piedmonts, through the Carapunco fault. b CC′ topographic profile showing the Mesada Lamedero positive relief structure, formed by the Carapunco and Blanquito Viejo faults. It is observed that the foothills of the Cumbres Calchaquíes are raised and folded over the foothills of the Sierra de Aconquija. 1: Loess deposits (Q). 2: Alluvial fan deposits (Q). 3: Alluvial fan deposits (Q). 4: Proterozoic basement. 5: Recent reverse faults. 6: Normal fault
Figure 10 shows the ravine raised on the left bank of the Tafí del Valle river. On the right bank of the Blanquito Viejo river, the river terrace is made up of light brown sandy silts, with intercalations of conglomerate lenses at the base, passing into a fanglomerate in the upper half of the profile. The conglomeradic lenses dip 30° to the west (265°/30°), product of the activity of the Blanquito Viejo fault (Figs. 9b, 10).
7.2.2 La Costa II, Los Cuartos
These zones are located in the southeast end of the Tafí valley, where the Tafí del Valle and Chasquivil faults converge (Figs. 2, 11). In this area, the Loma Pelada constitutes a positive element that divides the Tafí valley (Fig. 2). The foothill deposits of the Sierra de Aconquija are better developed in the Las Carreras valley, but not on the eastern edge of the Loma Pelada, which is where the piedmonts deposits of the Cumbres Calchaquíes will end up, against the Tafí del Valle River (Figs. 2, 11).
Image map (Sentinel image) of the southeast end of the Tafí valley to show recent structures in the foothills. H1 and H2: Accumulations, morphosedimentary units of the Holocene [30]. Pl: Pleistocene. Bm: Upper Proterozoic metamorphic basement. – Lower Paleozoic. Recent structures are shown in red and old structures in black. 1: Reverse fault. 2: Normal fault. 3: Transcurrent fault. 4: Lineaments. 5: Bedding strike and dip. 6: Anticline fold. F13 and F14 indicate the location of these figures
Quaternary structures were generated both in the foothills and in the mountain front of the Cumbres Calchaquíes (Fig. 11). In these areas we were able to make measurements of structures and verify the folds of the conglomerate banks of the outcrops. In other nearby areas such as Las Siringuillas, Las Nubes and La Angostura we do not find outcrops where structures can be measured, but the deformation of the alluvial fans adequately reflects neotectonic activity (Figs. 11, 12).
Photo showing the deformation of the piedmont by neotectonic processes associated with the Chasquivil fault. Hills oriented to the NNW are observed, made up of Pleistocene sediments, modifying the course of surface drainage, disturbing the H2 accumulations. Symbols are explained in Fig. 11
In the La Costa II área, the tectonic deformation of the Quaternary sediments occurs on the mountain front. Here we can see the cuspid zone of the deformed dejection cones, ridden by metamorphic basement, through a reverse fault dipping to the ENE (080°/80°) (Fig. 11). This fault corresponds to a reactivation of the Tafí del Valle and Chasquivil faults and the quaternary sediments would be represented by the H1 accumulations [30] (Figs. 2, 11). Other reverse faults (130°/88°, 280°/50°, 115°/80°), measured in metamorphic basement, also affect the quaternary sedimentary sequence. One of these faults (045°/90°) coincides with the Tafí del Valle fault (Fig. 11). Here, the quaternary faults are affecting the accumulations identified as H1, dated by Peña Monné and Sampietro Vattuone [30] between ca. 13000 and 4200 a BP (Fig. 11; Table 1).
In this area, rock falls by destruction of the terraces on the margins of the ravines are also observed, which allow evidence of recent tectonic activity. The discrete deformation (Table 2) associated with the recent seismic activity recorded in the area generates these neotectonic indicators.
Between the Los Cuartos and Las Siringuillas area, the piedmont landscape is crossed by structures with NW and NE strike and disturbed by hills of about 400 to 800 m in length and 140 m in width, with NNW strike (Figs. 11, 12). These hills are made up of Pleistocene sediments and are deforming the H1 and H2 accumulations (Figs. 11, 12; Table 1). Two anticline fold structures are observed, with NW and NE strike, which dispose the Pleistocene conglomerate strata dipping to the NE, SW and SE, NW, respectively (Fig. 13; Table 1).
View of folds in the foothills, the folded strata correspond to the Lomitas Pegadas Formation. a Anticline NW strike. b NE direction fold whose strata show strong dip
The faults have reverse displacement, dipping to the NW and NE (300°/45°, 080°/80°, 125°/70°, 115°/85°, 060°/84°). One of them (060°/84°) also evidenced dextral transcurrent movement, dividing one of the hills and generating a typical whale-back structure and a fault gap (Figs. 11, 14). Other structures are affecting the folds and H1 and H2 accumulations (Figs. 11, 12; Table 1).
View of a dextral transcurrent fault, with a vertical displacement component, affecting an outcrop of Lomitas Pegadas Formation of Pleistocene age. On the surface of the ground you can see the falt scar. At the bottom there is a terrace on the left bank of the stream, made up of H2 accumulations. The fault gap is visible in the stream bed. The hammer serves as a scale
The faults generated in the area between Los Cuartos are found in the cuspid part of the alluvial fans, near the mountain front and generate anticline folds in the Pleistocene deposits (Fig. 11, 13; Table 1). These structures and associated morphologies appear to be closely related to the Chasquivil fault. The layers of the H2 accumulations, dated by Peña Monné and Sampietro Vattuone [30] between ca. 4200–630 to BP are also affected by tectonic activity (Fig. 13a). The drainage network also shows tectonic activity, the channels are interrupted and diverted by the anticline structures and show recent incision in the H2 accumulations (Figs. 11, 12).
8 Discussion
The NE compression due to the convergence of the Nazca and South American plates produces deformation through movement in faults of diverse geometries. The deformation is influenced, among other things, by the rheology of the rocks and pre-existing faults [22, 40, 45,46,47, 67, 81,82,83].
Seismic rupture occurs when the elastic stress to which the rock is subjected exceeds its frictional resistance [53,54,55,56,57,58,59,60, 66, 67]. Most of the earthquakes recorded in the study area are of magnitude 3 and 4, which produce mean displacements of 1 and 3 cm (Table 2), respectively. The intense deformation of the foothills of the Tafí and Amaicha valleys shows a sustained tectonic activity over time, marked in the sediments of 630 BC and by recent seismic activity, which, despite being of low intensity, the small recurrent deformation and cumulative that it produces is capable of generating discrete deformations in the landscape, sometimes imperceptible to the human eye.
Recent earthquakes would be linked to one of the faults in the region (Tafí del Valle, Chasquivil, Aconquija, Periquillo, Río Muerto and Narváez faults and with the Los Pinos lineament) (Fig. 1).
Both in the Amaicha valley and in the Tafí valley, the tectonic deformation occurred in the foothills. The structures generated in the foothills are represented by reverse, normal and transcurrent faults and anticline folds (Figs. 3a, 6, 7, 8, 10, 13, 14). We interpret that the deformation occurs in the foothills because the sediments that fill these valleys have less cohesion and frictional resistance than the planes of the pre-existing faults that favor the dissipation of the seismic energy generated by tectonic activity. Consequently, the seismic energy is attenuated by the deformation of the pedemontane deposits (Figs. 7, 10, 12, 13) instead of being abruptly released by the breakdown of the basement rocks (Fig. 4).
As an example that materials with less cohesion and frictional resistance favor the dissipation of seismic energy and, consequently, tectonic deformation occurs in the first instance on these materials, we cite the Neogene tectonic deformation that occurred in the Cretaceous basins of northern Argentina and in the piedmont deposits of other regions. In the Cretaceous Rift basins, normal faults were reactivated as reverse faults [43,44,45,46] due to frictional resistance in the pre-existing faults planes is less than that of the rock of the sedimentary packages and also because the strike of these pre-existing faults is perpendicular to the Andean compression. The seismic profiles of the rift basins (Metán and Tucumán basins) show the reverse reactivation of cretaceous normal faults, with no new faults forming [45, 46]. A similar situation occurs in the western foothills of the La Candelaria range where a reverse, blind fault, vergent to the east, was generated taking advantage of the stratification planes of the tertiary sequence [73, 75] that offered less cohesion and frictional resistance.
But, in the study area and to the west of the Tucumán basin, the Cretaceous Rift faults did not have great development and large and deep basins were not formed (Las Cañas, Río Muerto, Periquillo, Chasquivil, Aconquija, Cumbres Calchaquíes and Portugues faults and the Los Pinos lineament). Only red sediments from the Lower Tertiary outcrops (Santa Bárbara Subgroup), there are no sediments corresponding to other secuences of the Cretaceous rift (Fig. 3) and the landscape is dominated by rocks from the Proterozoic basement. In addition, some normal faults inherited from the Cretaceous Rift (Los Corpitos, Infiernillo, Tafí del Valle, Las Cañas) were not reactivated, they were cut by the reverse neogenic and quaternary faults [6, 7, 22, 47].
Several seismic foci of magnitude ≥ 3 are located on the trace of the Chasquivil and Tafí del Valle faults (Fig. 1). This recent tectonic activity generates retrograde erosion, mobilizes and loosens the structural framework of the conglomerate deposits of the fluvial terraces and of the alluvial fans and of the regoliths of the metamorphic basement, triggering tumble falls and consequent landslides (Figs. 4, 11, 12, 13).
In the Tafí valley, the deformation of the Lomitas Pegadas Formation shows tectonic movements in the Upper Pleistocene [33]. In the Tafí valley the deformation of the foothills is greater than in the Amaicha valley, perhaps due to the fact that the valley fill consists only of deposits of quaternary alluvial fans, of less cohesion.
In the foothills of the Cumbres Calchaquíes, in the Tafí del Valle area, two structural zones are distinguished that show the differential propagation of seismic energy and consequently deformation. In the southeast sector of the valley, the deformation was directed by the Chasquivil fault and was confined by the Loma Pelada range to the west, and by the convergence of the Cumbres Calchaquíes and Sierra de Aconquija, to the southeast (Fig. 2). Consequently, in the foothills zone there were reverse and transcurrent faults and very tight folds; no evidence of displacement of the Carapunco fault was found in this area (Figs. 2, 12, 13, 14). The monotony of the foothills landscape is interrupted by folds and fractures. These tigger the diversion of the drainage channels whose recent incision is marked in the H2 deposits (Figs. 11, 12). Towards the northwest, the Tafí valley ends against the El Infiernillo pass, where once again the Sierra de Aconquija and the Cumbres Calchaquíes are held together by a partially buried igneous-metamorphic body (Fig. 2). In addition to the lower cohesion of the piedmont deposits, the seismic energy propagated and deformation taking advantage of the opening of the valley in the extreme northwest (Fig. 2). The sinistral component of the Carapunco fault generated an imbricated system of contractional faults with fan geometry (Figs. 2, 8) and the reverse component produced the thrust of the Cumbres Calchaquíes piedmont on the alluvial fan deposits of the Sierra de Aconquija (Fig. 10).
The hypotheses of Ruiz Huidobro [1], Ferreiro and Mon [2], González [3] and González and Barreñada [4] are verified, the Amaicha valley is delimited to the north by the Tafí del Valle normal fault and to the south by the Los Cardones fault (Figs. 2, 5, 6). In the Amaicha valley the neotectonic deformation of the piedmont is associated with the Los Cardones fault and other faults in the pedemontane area (Figs. 1, 2, 5, 6, 7).
9 Conclusions
The neotectonic deformation in the Tafí and Amaicha valleys occurred in the foothills of the Cumbres Calchaquíes through folds and fractures with NE and NW strike. Some fractures experienced horizontal movements in both directions and others had reverse displacements, retrovergent with respect to the Andean deformation (Fig. 2).
In the study region there are many earthquakes of magnitude ≥ 3 and together with some of magnitude ≥ 4 coincide with regional faults (Tafí del Valle, Chasquivil, Río Muerto, Los Pinos). This coincidence shows that these regional faults are active (Fig. 1).
The presence of two faults that limit the Amaicha valley is confirmed. To the north it limits with the Cumbres Calchaquíes through a normal fault that slopes towards the SW and has a horizontal dextral component (Fig. 2). To the south it is bounded by the Sierra de Aconquija through the reverse Los Cardones fault, vergent to the SW (Fig. 2). In the piedmont of this valley the Andalhuala Formation strata ride over the conglomerates of the Yasyamayo Formation and the quaternary gravels are also affected by these faults (Figs. 2, 7).
The Cumbres Calchaquíes ride over the Sierra de Aconquija through the Los Cardones and Carapunco faults (Figs. 2, 10). In the Mesada Lamedero the Carapunco fault has 176 m of fault offset (Fig. 10). The Carapunco fault also has a horizontal synestral component that is responsible for generating an imbricated system of contractional fractures, curved in plan, in the piedmont deposits of the Cumbres Calchaquíes in the Las Tacanas area (Figs. 2, 8). In the southeast end of the Tafí valley (La Costa II, Los Cuartos, Las Siringuillas, Las Nubes, La Angostura) the neotectonic activity has generated faults and folds in the foothills affecting the conglomerate sequences of the Pleistocene Lomitas Pegadas Formation (Figs. 11, 12, 13, 14). Tectonic activity in the piedmont of Cumbres Calchaquíes are affecting deposits dated 630 a BP.
The alluvial fan deposits in the piedmonts, as they have less cohesion and frictional resistance than the fillings of the pre-existing faults, absorb and attenuate the seismic energy propagated. On the other hand, the strike of the pre-existing faults and the arrangement of the stratification planes of the sedimentary sequence with respect to the Andean compression can be favorable to the reactivation of these planes.
In the study area, the normal faults of the Cretaceous Rift, with unfavorable orientations with respect to the Andean compression, were not reactivated (Las Cañas fault, Tafí del Valle fault). Even other N and NE strike faults (Infiernillo, Los Corpitos, Portugués) were cut by faults with opposite strike and senses. However, the reverse reactivation of the Chasquivil fault, with west vergence, produced the tilting towards the SE of quaternary conglomerate deposits (110°/12°) on the left bank of the Los Sosa River, in the La Angostura area, and of red sediments from the Paleogene, in the riverbed of the same river (Figs. 1, 2, 3, 11, 12).
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Acknowledgements
This work was carried out with the support of the National University of Tucumán and the financing of the Argentine-German University Center. The authors express their gratitude to Dr. Francésc Sabat Monserrat for the critical reading of the manuscript and his valuable suggestions. Also a special recognition to the reviewers, Dr. Lucio Pinotti and other, who with their criticisms and suggestions contributed to substantially improve the paper.
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Gutiérrez, A.A., Mon, R., Arnous, A. et al. Piedmont deposits as seismic energy dissipators, Sierras Pampeanas of Argentina. SN Appl. Sci. 3, 887 (2021). https://doi.org/10.1007/s42452-021-04874-0
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DOI: https://doi.org/10.1007/s42452-021-04874-0